Background: This proposal builds on our current NIH-Clinical Investigator Award (CIA) study of energy expenditure (EE) and body composition in prepubertal, 5-10 year old females. The proposed study will enable us to prospectively follow this group of children annually for 5 years to determine EE and body composition changes through puberty. We hypothesize that children with low total (resting and nonresting) EE and low rates of fat oxidation will predict relatively rapid weight gain during puberty. Objectives: 1) To examine prospectively the total EE (adjusted for body composition) and the rate of fat oxidation in children during different Tanner stages. 2) To examine the components of EE in children with more rapid weight gain and those with normal-weight gain during puberty. 3) To track in each child the variability in EE and fuel utilization during different Tanner stages. Design: In our on-going NIH-CIA study, 56 normal-weight prepubertal girls have undergone assessment of EE, body composition, biochemical profiles, and physical activity. We propose to reevaluate these children annually over a period of 5 years. Significance: This study should provide new insight into the contribution of variation in EE and body composition during different stages of puberty to long-term weight gain. Using state-of-the-art methods, this study will provide valuable information for understanding which factors predispose certain individuals to obesity during puberty. This information is essential to develop strategies for the prevention of obesity starting in childhood.